The major purpose of this proposal is to study the effects of actinic damage on DNA repair capacity in aging keratinocytes. Biochemical assays for repair replication of DNA will be employed, and in other studies Herpes Simplex Viruses (HSV) will be used as probes, to quantitate DNA repair. Initially, comparisons of the growth characteristics of HSV in these epidermal cells will be made. Subsequently, host cell reactivation of UV irradiated HSV-1 in sun-exposed and non-exposed keratinocytes will be determined to assess the capacity for repair of UV damage. Weigle type reactivation (WR) will serve as a quantitative measure of induced of induced DNA repair in keratinocytes, and will be compared in the sun-protected and exposed cells from the same individuals. This induced DNA repair pathway has been demonstrated to be error-prone in bacteria and eukaryotic cells. Since chronic sun exposure is related to the development of skin cancer and C linical signs of aging, error prone repair will be studied in actinically damaged keratinocytes of varying age. The assay systems to be used will quantitate thymidine kinase deficient HSV within the population of reactivated virus. The effects of actinic damage on that capacity of keratinocytes of varying age to perform excision-repair following exposure to UV light will be studied, employing density labeling with bromodeoxyuridine. This will provide a quantitative biochemical measure of repair of pyrimidine dimers in chronically sun-exposed and sun-protected epidermal cells throughout life.